Wow... so much... ignorance.
And it happens every time this subject comes up. (that isn't aimed at anyone in particular... I just find it a bummer that so few people learn about this stuff)
Weight doesn't change from lab ray.
Orange chicken doesn't add weight, or it wouldn't be so cheap. My pet would have eaten PPT's mascot if it were true though.
BBQ Porkwich DOES add weight. It'd still be quite cheap except that Bread is retired.
Pumpkin Neggs add 1-2 lbs and subtract 1-2 cm of height.
Earth faerie quests add a pound to whatever pet is fed.
There's a r99 item that increases height.
Neither height or weight affects anything.
Weight and height are initially determined by the pet's species upon creation. Neocolours used to have a statistical sampling of pet weights/heights, but I don't see the section I remember. There is a listing of height averages here:
http://www.neocolours.me.uk/petsizes.php
The heaviest pet is probably
Scowlserz It'd be mine if my account hadn't been frozen 2.5 years ago.